1. Field of the Invention
This invention, in various embodiments, relates generally to integrated circuit fabrication, particularly to methods for etching materials during integrated circuit fabrication and resulting structures.
2. Description of the Related Art
Integrated circuit fabrication conventionally includes forming a pattern of openings in a soft or hard mask and etching a material through the patterned mask to form openings in the material. In some applications, the etched material can, in turn, be used as a hard mask (or a second hard mask) for a subsequent pattern transfer to underlying materials. In damascene applications, the etched openings can take the form of, e.g., vias and/or trenches in insulating layers, which can be used to form various parts of an integrated circuit, including conductive contacts, interconnect lines and electrical devices such as capacitors and transistors.
Etching a material can involve performing a dry etch in which the material is exposed to a directional plasma, in which excited species are directed to the material at a tight distribution of angles. In principle, the dry etch forms uniform openings having relatively straight sidewalls. The excited species etch the material by forming volatile species with the material and/or by physically sputtering away the material, due to bombardment by the excited species.
Commonly etched materials include dielectrics, such as interlevel dielectrics. Openings in the dielectrics can be used to hold various conductive or semiconductive features in an integrated circuit, with the dielectric providing electrical insulation between the features.
Silicon oxide is a commonly used dielectric material, which can be formed in a variety of ways and can include various other constituents. A typical dry etch chemistry for silicon oxide-based materials includes hydrofluorocarbons, oxygen (O2) and an inert gas. With reference to FIG. 1, the etch chemistry can be directed as plasma excited species through openings 10 in a masking layer 20 to etch a silicon oxide layer 30. With reference to FIG. 2, while the flow of the plasma excited species is predominantly vertical, the paths of some plasma excited species have a horizontal component which can cause etching of the sidewalls. This etching results in the formation of bowed sidewalls and, as a result, is commonly referred to as “bowing.” The resultant thinning of insulating material between vias or trenches can lead to, among other things, breakage, shorting between conductive elements filling the vias/trenches or parasitic capacitance.
With continued reference to FIG. 2, carbon from the hydrofluorocarbons of the etch chemistry can deposit and polymerize in the openings 50 during etching, thereby forming polymer films 60. In general, the deposition occurs predominantly adjacent the walls of the masking layer 20, near the upper parts of openings 50. The polymer films 60 form a so-called neck at the thickest parts of the polymer films 60. In some cases, the polymer films 60 can grow so thick that they block or plug the openings 50. Typically, however, the openings 50 remain open and these necks can be beneficial, since they can form a “shadow” over the sidewalls 40, thereby protecting the sidewalls 40 from etching, thereby decreasing bowing.
While desirable for protecting the sidewalls 40, polymer films 60 with thick neck regions can also block the flow of some etchants into the openings 50; more etchant reaches the middle parts of the bottom of the openings 50 than the peripheral parts, causing material at the middle parts to be preferentially removed. As a result, the openings 50 can taper as they progress further into the layer 30. If the polymer film 60 deposits asymmetrically in the openings 50 or in different amounts between different openings 50, the tapering can be non-uniform, causing the formation of non-uniform openings 50. It will be appreciated that the formation of straight sidewalls 40 and uniform openings 50 are typically desired in integrated circuit fabrication to allow for, e.g., predictability, reliability and uniformity in the properties of the final product. Minimizing tapering by forming thinner polymer films 60 in the neck regions, however, may provide inadequate protection of the sidewalls 40, causing excessive bowing.
Accordingly, there is a need for methods and structures that allow effective control of the profiles of etched openings.